The present invention pertains to an air flow rate measuring device (air flow rate meter) for use with an internal combustion engine. More particularly, the invention relates to a vortex-type flow meter having improved transient characteristics.
A von Karman type flow meter and a swirl type flow meter have recently been used as air flow rate meters. Both types of flow meter provide an output signal having a frequency approximately proportional to the flow rate of intake air. Such an output signal accurately representing an air flow rate is obtainable over a fairly wide air flow rate range. A typical example of such an output signal is a pulse signal which has a high level in response to the detection of a vortex. Since the formation of vortices follows quickly transient variations in the air flow velocity, the output signal also responds to the air flow velocity quickly. However, it is impossible to indicate an actual air flow rate with a time delay shorter than one period (an interval between pulses) of the output signal. Accordingly, the response of the output signal is still not as fast as desired. Further, in the case where each period of the output signal is measured to detect the air flow rate, if the actual flow rate varies greatly during one period of the output signal, the resultant rate indicated by the output signal is much different than the actual flow rate. There is another difficulty in that a stable output cannot be obtained without an averaging process where an average over a number of periods of the output signal is taken because the period of the output signal tends to fluctuate due to distortions in the shapes of the vortices. This defect is fatal to a system in which the fuel flow rate or the exhaust gas of an automobile is controlled on the basis of the flow rate of intake air. Further, since the conventional vortex flow meter operates to detect the frequency of formation of vortices, it is disadvantageous in that, upon occurrence of turbulence, the formation of vortices is distorted resulting in an error in the output signal.
It is the primary object of the present invention to eliminate the above-described drawbacks accompanying prior art flow meters.